21. VOLCANIC GLASS IN SOME DSDP LEG 36 CORES1

D.H. Elliot and C M . Emerick, Institute of Polar Studies andDepartment of Geology and Mineralogy, The Ohio State University, Columbus, Ohio

The persistent occurrence of minor to trace amountsof volcanic glass in the coarse fraction of the sedimentrecovered at Site 329 prompted an attempt to recoverthe sand-sized glass and establish its composition.Samples from other sites were also examined, but theabundance in those cores is similarly low.

The siliceous organisms present dictated that with-out the use of elaborate procedures the only method ofseparation of the glass would be hand picking of thecoarse fraction. The samples were washed and thegreater than 62.5 µm fraction was collected and handpicked; in the sand-size range the glass is colorless andtransparent and is readily distinguished by its shardform (Figure 1).

The composition has been estimated by determina-tion of the refractive index by interference microscopy.The equipment and technique were developed by E.G.Ehlers and C M . Cocker of the Department of Geologyand Mineralogy at the Ohio State University (Cocker,1974). The equipment used allows the determination ofthe refractive index to ±0.005. (It has since transpiredthat a similar technique has been developed in-dependently by E.C.T. Chao [unpublished] and gives aprecision of ±0.0002.)

The results are compiled in Table 1 and Figure 2. Allexcept two samples fall within the range 1.517 to 1.502,corresponding to Siθ2 percentages of about 64% to72%. One of the two exceptions has an R.I. of 1.523(SiO: a 62%), and the other an R.I. of 1.492 (SiO2

75%). The bulk of the analyzed glass falls within therange 1.517 to 1.502, but the composition can be in-ferred only approximately from the refractive indexbecause of the natural variation in rock types as well aspossible hydration of the glasses. The silica percentagerange, 64% to 72%, corresponds to rocks in the dacite torhyolite compositional range.

The occurrence of volcanic glass shards in sedimenton the Falkland Plateau can be attributed to direct air-fall and settling through the water column, with an un-determinable amount of transport by bottom currents.The prevailing winds in the latitude of the FalklandPlateau are the westerlies, which would bring volcanicash from the general area of southern South Americaand the northern part of the Antarctic Peninsula(Figure 3). Under certain conditions, which occurrather infrequently, it is possible to derive ash from theSouth Sandwich Islands which lie to the southeast. Thisseems unlikely for the majority of core samples becausevolcanic rocks of those islands have yielded K-Ar ages

'IPS Contribution No. 286.

no older than about 4 m.y. (Baker, 1968) and arepossibly no older than 8 m.y., based on the sea-floormagnetic anomaly data (Barker, 1972a). Basalt andsubordinate andesite make up about 95% of the ex-posed rocks with dacite and rhyolite forming theremainder; explosive activity does not appear to havebeen significant in the construction of the island arc(Baker, 1968).

The alternatives, then, are the northern AntarcticPeninsula and southern South America. The Tertiary,Quaternary, and Recent volcanic rocks of the northernAntarctic Peninsula occur in two general areas, theSouth Shetland Islands and the James Ross Islandregion. The Tertiary volcanic activity in the SouthShetlands is rather poorly documented, however, it isprobable that the upper Tertiary rocks are related tothe subduction zone active from at least 20 m.y. toabout 8 m.y. ago, based on the sea-floor magneticanomalies in the Drake Passage (Barker, 1972b). TheTertiary volcanic rocks are largely basaltic andesiteswith only one felsic lava recorded (Hawkes, 1961a). Themore recent activity at Deception, Penguin, and Bridge-man islands appears to be related to rifting in theBransfield Strait. The volcanism is alkaline with amarked soda enrichment; compositions range fromhawaiite through mugearite to trachyte (Baker, 1972)(olivine basalt to trachyte, Hawkes, 1961b). The age ofinception of this period of volcanism is uncertain,however none of the rocks dated so far give pre-Pleistocene ages (C.H. Shultz and R.J. Fleck, personalcommunication). The volcanism of the James RossIsland area (Nelson, 1966; Baker et al., 1973) is alsomildly alkaline in character and the products consistlargely of palagonite breccia and olivine basalt lava(hawaiite, Baker, 1972). These rocks have given K-Ardates from 4.6 m.y. to less than 1.0 m.y. (Rex, 1972),and like the active or recently active centers north of theBransfield Strait, are probably associated with rifting.There is a general paucity of felsic lavas in the northernAntarctic Peninsula, although the lack of systematicsurvey and petrologic examination may partly accountfor their apparent absence; certainly felsic lavas cannotform a very significant part of the volcanic record.

Tertiary strata cover much of Patagonia from about40° southwards. Lower Tertiary rocks of volcanicorigin are known from a number of areas: Eocene tuffsand lavas of basic and acid composition are recorded inthe Lago Nahuel Huapi region (Dessanti, 1972); tuffsand tuffaceous rooks constituting the "Serie Rio-dacitica" crop out in northwestern Chubut Province(Lesta and Ferello, 1972) and are believed to be Eoceneand equivalent to the "Serie Andesitica" in the cor-

871

00

to

i

Figure 1. Scanning electron microscope photographs of volcanic glass shards from Leg cores (a) 329-3, CC; (b) 329-10, CC; (c) 329-13, CC; (d) 329-13, CC.

VOLCANIC GLASS

TABLE 1Refractive Index of Volcanic Glass

in Some Leg 36 Core Samples

Sample No.

Site 327

327A-5-4, 110

327A-9-1, 145

Site 328

328-2, CC328-3, CC328A-1-1, 79-81328A-1-2, 54-56328B-1-1, 147-149328B-2-1, 54-56328B-2-3, 84-86328B-34, 54-56328B-6-2, 54-56

Site 329

329-2-3,4-6329-2-3, 65-67329-24, 21-23329-2-4, 54-56329-2-5,54-56329-2-6, 54-56329-2, CC329-3-1,80-82329-3-4, 80-82329-3, CC329-4-1,68-70329-4-5, 80-82329-4, CC329-5, CC329-6, CC329-7, CC329-8, CC329-9, CC329-10, CC329-11, CC329-12, CC329-13, CC329-14, CC329-19, CC329-21, CC329-23, CC329-26, CC329-27, CC

Site 330

330A-1-3, 51-53

Age

Late Paleocene toearly EoceneLate Paleocene toearly Eocene

Early PlioceneLate MiocenePleistocenePleistoceneLate PlioceneLate MioceneMiddle MioceneLower MioceneOligocene

Late MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneLate MioceneMiddle MioceneMiddle MioceneMiddle MioceneMiddle Miocene

Oligocene to Recent

R.I.

1.507

1.514

1.5041.5051.5231.5131.5011.5131.5091.5061.517

1.5021.5041.5071.5111.5051.5161.5171.5121.5141.5161.5051.5081.5031.5101.5051.5121.5051.5091.5021.5081.5081.5131.5101.5091.5091.5041.4921.505

1.512

dillera at least as far south as Lago Fontana and to the"Tobas de Sarmiento" in the region of the Gulf of SanJorge; and in northwestern Santa Cruz Province the"Serie Andesitica" crops out again and consists of in-termediate to acid volcanic rocks (Leanza, 1972)although here it is assigned a ?Paleocene-Eocene age.The Patagonian Formation, which covers large areas ofcentral and eastern Patagonia, contains both marineand nonmarine beds in which volcanic detritus in theform of tuffs and tuffaceous rocks has been observed(Feruglio, 1949). It was formerly thought to be in partMiocene, but recent work suggests it is Eocene to earlyOligocene in age (Camacho, 1974). Younger Tertiary

strata with a volcanic component include the MioceneSanta Cruz Formation which crops out extensively inSanta Cruz Province between the foothills of the Andesand the Atlantic Coast (Russo and Flores, 1972). TheSanta Cruz Formation contains tuffs and tuffaceoussediments in which pumice fragments have been identi-fied (Feruglio, 1949). Younger Miocene strata appearto be confined to the Collón Curá Formation; it cropsout near Lago Nahuel Haupi and includes tuffs ofdacite to rhyolite composition in which both volcanicglass and pumice fragments have been recognized(Dessanti, 1972). The same formational name has beenapplied to a sequence of rocks in the southwestern-mostSanta Cruz Province which consists of conglomerate,sandstone, shale, and tuff (Leanza, 1972).

In southern South America late Cenozoic to Recentvolcanic activity is concentrated along the Andes andits eastern flank in Patagonia. The Patagonian basaltsare widely distributed; the thicker sequences are all ad-jacent to the Andes, whereas toward the Atlantic Coastthe sequences are much thinner and occur irregularly.The basalts are described as alkaline and all are floodbasalt sheets; no intermediate or acid compositionshave been recorded. The age of the Patagonian basaltsis uncertain, but is known to span at least 3.5 m.y. to1.0 m.y. (Mercer et al., 1975). South of latitude 44°S,where the Chile Rise abuts the South American conti-nent, active or recently active volcanoes occur onlysporadically; north of that latitude volcanoes are abun-dant. Information on the composition of the rocks ofthe southerly stratovolcanoes is sparse (Casertano,1963; Fuenzalida, in press); andesitic compositionsappear to predominate. Most of the stratovolcanoesnorth of the Chile Rise have been documented byMoreno (1974) and consist of a full range ofcalcalkaline rocks from basalt to rhyolite, althoughacidic compositions are subordinate.

The distribution in time of the tuffaceous strata inPatagonia and the active and recently active strato-volcanoes of the southern Andes covers the same inter-val as that of the core samples from the FalklandPlateau, however there is little which specifically helpscorrelation because of the lack of sufficient data on theage and composition of the rocks.

The source of the volcanic glass in cores examinedhere probably comes ultimately from the southernAndes and from explosive eruptions possibly co-magmatic with the widespread Tertiary graniticplutons. Radiometric dating of plutonic rocks in thesouthern Andes is sparse and the only Tertiary dataavailable is that of 12 ±2 m.y. for the Cerro del Paineintrusion not far north of the Straits of Magellan(Halpern, 1972).

The tentative identification of the southern Andes asthe most likely source region raises some questionabout the magnitude of eruptions necessary to trans-port sand-sized material for 2-3000 km. The factors in-volved in atmospheric transport of tephra have beendiscussed by Shaw et al. (1974), and include the magni-tude of the eruption and hence the height to which theparticles of any given size range are driven, and theprevailing wind velocities and hence the distance traveled

873

D. H. ELLIOT, C. M. EMERICK

327A/5/4/II09/1/145

328/2/cc328/3/cc

328A/I/1/79-81I/2/54-56

328B/I/I/I47-I492/1/54-562/3/84-863/4/54-566/2/54-56

329/2/3/4-62/3/65-672/4/21-232/4/54-562/5/54-562/6/54-562/cc3/1/80-823/4/80-823/cc4/1/68-704/1/80-824/cc5/cc6/cc7/cc8/cc9/cc

10/ccll/cc12/cc13/cc14/cc19/cc21/cc23/cc26/cc27/cc

330A/1/3/51-53

•490 1-495 •500 1-505 1-510

Refractive Index

520 •525

Figure 2. Refractive index of volcanic glass shards from Leg 36 cores.

by those particles before settling out. Shaw et al.(1974) discussed particles in the range 11-88 µm whichincludes most but not all of the size range (>62.5 µm)studied in the Leg 36 cores. Using 72 µm as a workinggrain size, it is clear from fig. 6 of Shaw et al. (1974)that for travel distances of 2-3000 km, that eruptions ofgreat magnitude and winds stronger than those of theNorth Atlantic at 50°N are both required. TheFalkland Plateau and the southern Andes lie in thetrack of the prevailing westerlies of the southern mid-latitudes and therefore very strong winds are to be ex-pected. The wind velocity profile at about 50°S, thelatitude of interest here, shows that velocities may be upto one and a half times those of the equivalent latitudein the northern hemisphere (Lamb, 1972) and allowsuse of the theoretical models developed by Shaw et al.(1974). In this, a wind velocity one and a half times that

of the North Atlantic at 50°N is used; if the windvelocity is not of that magnitude, the cloud height andmagnitude of eruption would have to be increased ac-cordingly. Thus the height of the cloud top, at one anda half times the wind speed, would have had to be in ex-cess of 25 km (80,000 ft), indicating for sand-sizedtephra eruptions of considerable magnitude (paleoex-plosivisity of >7 megatons, fig. 7, Shaw et al., 1974) totravel 2000 km (Sites 327, 329, 330) and.considerablygreater magnitude to travel the extra 700 km to Site328.

Assuming the results give approximately correctfigures, it is clear that the source of the shards and theirmode of transportation need to be re-evaluated. Thereis some question about the former disposition of thecontinental blocks around the Scotia Sea because muchof the sea-floor magnetic anomaly data cannot be iden-

874

VOLCANIC GLASS

60°W/

/ ,

/ j/ }

50° W40°W

{/Bαthymetric contours'

1,000 m—- 3,000 m

* Active volcano

0 km IX>OO

\ South Atlantic Ocean

V"-•-+-!TndsV£? / F0lkl0nd -i‰wß1 .328

y " ×

Straits ofMagellan

'-/ / r-'-:"× X \ V ) Georgia

-Λ, Scotia Sea V A

/ ~•-i

90° W\

80° W\

i ** \

''' £ Sandwich

islands '

Peninsula D - Deception IslandB- Bridgeman Island

40βW 30°W 20° W

Figure 3. Location map for possible sources of volcanic glass shards from Leg 36 cores.

tified yet with the known geomagnetic polarity timescale. The South Sandwich Islands are probably noolder than 8 m.y., however a precursor to that islandarc may have existed although no substantial readilyidentifiable volcanic edifice was built. It is most unlike-ly to have been the source for tephra of acidic composi-tion. The Drake Passage started to open about 20 m.y.ago, and the South Shetlands appear to contain theonly associated continental volcanic record. Becauseevents earlier in the Tertiary than that cannot beevaluated, doubts must remain about the source of theshards in older core material. However, the southernAndes appear to be the most likely source area provid-ed oceanic currents have not displaced the sand-sizedmaterial any great distance. Bottom currents at Site 328flow from the southeast, a direction in which there is noadequate source. Bottom currents across the FalklandPlateau flow in a northerly direction and it is possiblethat there has been transport from a more westerlysource, however the source areas that become available,principally the Antarctic Peninsula, are again unlikely.Furthermore the prevailing westerlies would not con-

sistently bring ash from the southern Andes into theScotia Sea for subsequent transport by ocean currents.

Therefore, explosive volcanic eruptions in thesouthern Andes are considered to be the most likelysource for the shards, and atmospheric transport themost likely mode of dispersion.

ACKNOWLEDGMENTS

We are grateful for the assistance given by CM. Cockerwith the interference microscopy and R.F. Markley with theSEM photographs. E.G. Ehlers reviewed the manuscript.

REFERENCESBaker, P.E., 1968. Comparative volcanology and petrology of

the Atlantic island arcs: Vole. Bull., v. 32, p. 189., 1972. Recent volcanism and magmatic variation in

the Scotia Arctic. In Adie, R.J. (Ed.), Antarctic geologyand geophysics: Oslo (Universitetsforlaget), p. 57-60.

Baker, P.E., González-Ferran, O., and Veraga, M., 1973.Paulet Island and the James Ross Island Volcanic Group:Brit. Antarctic Surv. Bull., v. 32, p. 89.

875

D. H. ELLIOT, C. M. EMERICK

Barker, P.F., 1972a. A spreading centre in the East ScotiaSea: Earth Planet. Sci. Lett., v. 15, p. 123.

, 1972b. Magnetic lineations in the Scotia Sea. InAdie, R.J. (Ed.), Antarctic Geology and geophysics: Oslo(Universitetsforlaget), p. 17-26.

Camacho, H., 1974. Bioestratigrafia de las formacionesmarinas del Eoceno y Oligoceno de la Patagonia: Anal.Acad. Ci. Ex. Fis. Nat. Buenos Aires, v. 26, p. 39.

Casertano, L., 1963. Catalogue of the active volcanoes of theworld including solfatara fields; Part XV: Chilean Con-tinent: Internatl. Volcanol. Assoc, Naples, Italy.

Cocker, M., 1974. A new method for the determination ofrefractive indices using the interference microscope. MSthesis, the Ohio State University, Columbus, Ohio.

Dessanti, R.N., 1972. Andes Patagonicos Septentrionales. InLeanza, A.F. (Ed.), Geologia Regional Argentina: Cor-doba, Argentina, p. 655-687.

Feruglio, E., 1949. Descripción Geológica de la Patagonia.Tomo II: Buenos Aires, Ministerio Industria y Comercio,Direccion General de Yacimientos Petroliferos Fiscales.

Fuenzalida, R., in press. The Hudson Volcano: Vole. Bull.Halpern, M., 1972. Geologic significance of isotopic age

measurements of rocks from Tierra del Fuego (Chile): Am.Geophys. Union Trans., v. 53, p. 182.

Hawkes, D.D., 1961a. The geology of the South ShetlandIslands: I. The petrology of King George Island: FalklandIslands Dependencies Survey, Sci. Rept., v. 26, p. 1-28.

, 1961b. The geology of the South Shetland Islands:II. The geology and petrology of Deception Island:Falkland Islands Dependencies Survey, Sci. Rept., v. 27,p. 1-42.

Lamb, H.H., 1972. Climate: Present, Past and Future: Lon-don (Methuen & Co.).

Leanza, A.F., 1972. Andes Patagonicos Australes. In Leanza,A.F. (Ed.), Geologia regional Argentina: Cordoba, Argen-tina, p. 689-706.

Lesta, P.J. and Ferello, R., 1972. Region Extra Andina deChubut y norte de Santa Cruz. In Leanza, A.F. (Ed.),Geologia regional Argentina: Cordoba, Argentina,p. 601-653.

Mercer, J.H., Fleck, R.J., Mankinen, E.A., and Sander, W.,1975. Southern Patagonia: Glacial events between 4 m.y.and 1 m.y. ago. In Suggate, R.P. and Cresswell, M.M.(Eds.), Quat. Stud. Roy. Soc. New Zealand Bull.: v. 13,p. 223-230.

Moreno, H., 1974. Andean volcanoes of central-south Chile:Guide book for Excursion D-3, IAVCEI Symp. on An-dean and Antarctic volcanology problems, Santiago,Chile, September 1974.

Nelson, P.H.H., 1966. The James Ross Island VolcanicGroup of northeast Graham Land: Brit. Antarctic SurveySci. Rept., v. 54, p. 1-62.

Rex, D.C., 1972. Potassium-argon age determinations onvolcanic and associated rocks from the Antarctic Penin-sula, and Dronning Maud Land: In Adie, R.J. (Ed.), Ant-arctic geology and geophysics: Oslo (Universitetsfor-laget), p. 133.

Russo, A. and Flores, M.A., 1972. Patagonia Austral ExtraAndina. In Leanza, A.F. (Ed.), Geologia regional Argen-tina: Cordoba, Argentina, p. 707-725.

Shaw, D.M., Watkins, N.D., and Huang, T.C., 1974. At-mospherically transported volcanic glass in deep seasediments: theoretical considerations: J. Geophys. Res.,v. 79, p. 3087-3094.

876